During the long operation period of a transformer, partial discharge may occur at some weak locations of its internal insulation under high intensity fields, leading to degradation of the insulation performance. With the long-term effect of severe partial discharge, it may even cause breakdown, and thus damage transformer. With respect to a transformer, especially an extra high voltage transformer, in order to check its insulation, a test of ACLD (AC voltage long-duration) with partial discharge must be carried out to examine insulation performance. In the on-site test, it is necessary to use a simple and reliable power supply system for the test to apply voltage to the transformer samples according to test requirements, while effectively suppressing or avoiding interference of the test power supply system itself, so that the interference of the test power supply system itself may meet the requirement of the partial discharge test. Because extra high voltage transformer substations usually have complex electromagnetic environments and a lot of outside interference sources exist, the test power supply systems in the prior art themselves have higher background interference level, unable to meet test requirements of extra high voltage transformers. Besides, the test power supply systems in the prior art have less capacities, usually, only adequate to tests for the transformers at level of 500 kV or less. However, when the capacity increases, the startup current of the motor may exceed a current threshold allowed by transformer substations, which therefore is unsuitable for extra high voltage transformers (e.g., 1000 kV and above transformers).